It is well-known in the art that the rate of dissolution of an alloying metal in the molten metal to be alloyed is the limiting factor in the production of such alloys and that for this reason it is desirable to reduce to a minimum the time required to add and dissolve the additive metal. In the production of manganese alloys of aluminum, for example, manufacturers are willing to accept slightly higher manganese cost and somewhat lower manganese recovery in the alloy in order to increase the rate of dissolution of the manganese in the aluminum. Manganese-aluminum alloys are important materials and, indeed, constitute a large proportion of all aluminum products. Alloys of this type, other than so-called master alloys, generally contain no more than about 1.5% manganese, by weight, although alloys containing up to 2 to 3% manganese may be useful for some purposes. Master alloys, which are intended to be dissolved in molten aluminum to make ordinary manganese-containing aluminum alloys, may contain from about 4 to 30% manganese. As a rule, however, lesser amounts of manganese, i.e., from 1.5% down to as little as about 0.01% are employed in commercial aluminum alloys. For example, type 3003 aluminum-manganese alloy, which contains from about 1.0 to 1.5% manganese, retains the high corrosion resistance of pure aluminum, but has much greater strength than commercial pure aluminum and also exhibits excellent forming and welding properties which adapt it for use in a wide variety of applications, such as aluminum foil and extruded shaped articles. Type 5056 manganese-aluminum alloy, which contains about 0.01% manganese, is a well-known example of a low manganese type of aluminum alloy.
The direct addition of manganese metal to molten aluminum is difficult, due to the fact that the melting point of manganese (1245.degree. C.) is much higher than the melting point of aluminum (660.degree. C.). Moreover, the rate of dissolution of metallic manganese in molten aluminum is normally very slow. As might be expected, in general, the smaller the particle size of the manganese metal, the faster its rate of dissolution in aluminum. For example, manganese chips dissolve in molten aluminum more rapidly than larger lumps of the metal. Despite this observation, it is not possible to pursue this advantage to its ultimate conclusion and simply add finely powdered manganese to molten aluminum. This is due to the fact that when powdered manganese is added to a bath of molten aluminum it floats on the surface and is sintered to a hard crust, with the result that much of the manganese is oxidized and fails to be recovered as manganese metal in the final alloy. For this reason, powdered manganese has previously been added to molten aluminum chiefly in the form of briquettes formed from mixtures of powdered manganese with powdered aluminum. While such composite powdered manganese-aluminum briquettes provide better results than powdered manganese alone, they have not proven entirely satisfactory. Briquettes composed substantially entirely of manganese powder have been found entirely unsatisfactory since they do not dissolve in molten aluminum. More recently, however, it has been discovered that powdered manganese may be added to molten aluminum together with a flux without undue sintering of the manganese.
In view of the above, the most usual means of adding manganese metal to aluminum has been to prepare an aluminum-manganese master alloy containing from about 4 to 30% manganese by weight. Such master alloys have the advantage of dissolving relatively rapidly in molten aluminum and also provide homogeneous distribution of the manganese throughout the aluminum bath. Despite these advantages, such master alloys have presented handling and storage problems for both users and producers and have the further disadvantage of being uneconomically high in cost. Therefore, a need has long existed for a simple, economical method for adding manganese metal directly to molten aluminum in such a way as to provide rapid dissolution of the manganese in the aluminum. The foregoing specific illustrative comments are also generally applicable to the addition of chromium and other additives to aluminum and are also generally applicable to the addition of any alloying metal to any molten metal.